Experiments are proposed to determine the structure of the 300 angstroms chromatin filament and to determine which groups on nucleosomes interact to stabilize it. The 300 angstroms filament (in which nucleosomes are thought to be helically packed) is a specially important stage in chromosomal folding, for several reasons: (a) it is the folded state in which most chromatin is maintained during interphase of the cell cycle; (b) it serves as the fundamental structural unit for further folding to produce meiotic symaptonemal complexes and metaphase chromosomes; and (c) regional unfolding of the 300 angstroms filament is associated with transcription, and is likely to be required for replication as well. Recent work has provided x-ray diffraction patterns from partially oriented samples of 300 angstroms filaments. These patterns distinguish definitively between previously published models. It is proposed here to continue and extend this work, in two ways. Various methods will be used to get improved orientation of samples for x-ray diffraction, and to extract additional important information, by x-ray scattering, from samples which can be prepared by current methods. The results of these studies will be used to deduce an improved model for the structure of the 300 angstroms filament. Of particular interest are the angles of rotation and tilt of the nucleosomal disks, and the location of linker DNA and of H1. A parallel study will combine biochemical modifications of chromatin with classical and dynamic light scattering, electron microscope, and x-ray solution scattering studies, to determine which groups on nucleosomes interact to stabilize the 300 angstroms filament, and which interact to stabilize the further folding of 300 angstroms filaments. From the results of these experiments, mechanisms by which the cell might regulate these processes will be proposed and subsequently tested.